The NTP completed long-term ozone studies in F344 rats and B6C3F1 mice. There was no evidence of carcinogenic activity in F344/N rats, however there was equivocal and some evidence of carcinogenic activity in male and female B6C3F1 mice respectively. Based on findings in female mice, s study was designed to determine if genetic alterations in the K-ras gene, shown to be important in human lung cancer, may play a critical role in the biology of ozone-induced lung neoplasms. The objectives of this study were to (1) evaluate benign and malignant lung neoplasms for mutations in the K-ras gene at codon 12, 13 and 61, (2) determine if the frequency and spectra of K-ras mutations were specific for ozone-induced neoplasms or similar to spontaneous neoplasms and (3) determine if specific K-ras mutations correlated with the size and morphology of lung neoplasms. Female B6C3F1 mice were exposed to 0, 0.5 or 1.0 ppm ozone for 24 and 30 months. K-ras mutations were identified by single stranded conformation analysis (SSCA) and confirmed by direct sequencing of polymerase chain reaction amplified DNA isolated from formalin-fixed paraffin-embedded neoplasms were similar to those of spontaneous lung neoplasms. However, a higher frequency of K-ras mutations were observed in ozone-induced neoplasms (73%), as compared to lung neoplasms from air controls (33%) or the historical data base for spontaneous lung neoplasms (24%). Specific K-ras mutations did not appear to correlate with the size or morphology of lung neoplasms. The data suggest that ozone exposure may have a promotional effect on background K-ras mutations in B6CeF1 mice. Alternatively, ozone exposure may enhance oxidative stress and thus enhance endogenous DNA damage. Treatment of animals with ozone was conducted on contract # NO1-ES-95239.